The present invention relates in general to a humidity sensor alarm unit, and it more particularly relates to a fail-safe alarm unit which activates an alarm circuit when the relative humidity changes to an out-of-limit condition or when a malfunction such as a power failure or cut cable occurs.
Electrical humidity sensors have been employed for detecting relative humidity. Such sensors employ hygroscopic film which changes its electrical resistance rapidly with minute changes in moisture conditions. While such humidity sensors have been employed successfully for many different applications, it would be highly desirable to have a humidity sensor detecting an out-of-limit humidity condition or when certain malfunction conditions occur, such conditions including the humidity sensor circuit being shorted or opened. In this regard, the alarm signal should be generated when a cable is cut or the humidity sensor is missing. Also, the alarm signal should be generated when a power failure occurs, or when any circuit component of the alarm unit should fail.